The present invention relates to a process and a device for deaeration of liquid, notably of paper pulp.
Paper pulp made from used paper generally involves, after the treatments of disintegration and primary purification, a stage of brightening the pulp by extraction of the pigments and inks, also called deinking.
This deinking is normally carried out by flotation: The paper pulp is injected under pressure into a vat through mixing injectors which form a foam, the bubbles of which catch the inks. The foam is then separated from the rest of the pulp by slow or fast flotation depending on the device used, and discharged.
This technique of flotation by production of foam can be used also for small and light-weight contaminants other than the ink.
However, while this technique gives good results as to the quality of the deinking of the pulp, it presents disadvantages in the treatment of the extracted foam.
In fact, after its removal the foam must be broken, this is, deaerated, firstly so as to reduce the large volume that it occupies, and then to separate the air from the pulp fraction it contains in order to recycle it.
The foam is generally destroyed in known separators. However, the collected liquid still presents a large amount of air in the form of very numerous very small bubbles; the density of the pulp is on the order of 0.7 to 0.5.
And this liquid which is to be re-used is difficult to transport precisely because of this presence of air: Pockets of air form in the pipes, preventing the use of classic pumps. It is then necessary to provide expensive volume displacement pumps.